The present invention relates to printers and more particularly to a novel print head for impact printers of the dot matrix type utilizing novel replaceable plastic bearings.
Dot matrix printers are typically comprised of a plurality of linearly aligned solenoid driven print wires. The print head is moved across a paper document and selected ones of the solenoids are energized to drive their associated print wires against an inked ribbon to form dot column patterns at closely spaced intervals along the line of print. In one typical embodiment, the print head comprises seven print wires and five dot columns are utilized to collectively form a single character whereby each character is formed within a 5 .times. 7 dot matrix. Selective energization of the solenoids permits the generation of alphabetic and numeric characters, punctuation symbols and the like.
Assuming a 132 column printer, i.e., a printer capable of printing 132 characters per line of print with each character formed within a 5 .times. 7 dot matrix, so that five dot columns are utilized for each character, each individual solenoid may be caused to function 660 times per printed line. In the formation of graphic patterns, each solenoid may be caused to operate 792 times per line of print.
The print wires are typically spaced about 0.006 inches from the inked ribbon and paper document. The total distance to the platen is approximately 0.015 inches. The forward ends of the print wires are spaced from the paper document less than 0.015 inches to allow the platen, paper document and ribbon to absorb some of the impact.
Present day printers are capable of printing at the rate of 330 characters per second and 125 lines per minute (for lines of 132 character length). In order to achieve these print speeds, the print wires must be capable of being accelerated from a rest position to a velocity sufficient to form a dot on the paper and return to its rest position in less than one millisecond. Acceleration rates on both impact and retraction and the impact forces occurring during dot printing impart continuous and significant sidewise forces upon the bearing which causes the conventional jewel bearings typically formed of ruby or sapphire to fracture and chip due to side loading forces thus shortening the life of the jewel and hence the print head structure. Also the present design makes the head substantially useless once the bearings are damaged or broken.